Method of coating metal

ABSTRACT

A method of coating a refractory metal body consisting essentially of Mo, Ta, W or Ti, comprises the steps of (1) cleaning the body, (2) spraying molten platinum group metals (e.g. Pt, Pd, Rh, or their alloys) on to the surface, (3) plastically deforming the coating (by shotpeening, pressing or rolling), and (4) heat treating the coated body at 600-1400 DEG  C. for a time sufficient to promote solid diffusion of the coating (e.g. 1-10 hours).  These steps (1)-(4) are repeated until a sufficient thickness of coating metal has been built up.  The heat treating step may be carried out in dry hydrogen.

United States Patent 3,066,042 Patented Nov. 27, 1962 3,066,042 METHODOF COATING METAL Joseph Ogden, Maywood, N.J., assiguor, by inesneassignments, to Engelhard Industries, Inc., Newark, N.J., a corporationof Delaware N Drawing. Filed Nov. 27, 1959, Ser. No. 855,508 1 Claim.(Cl. 117-65.2)

The present invention deals with a method of coating metal and moreparticularly with a method of coating refractory metal with aplatinum-group metal.

It is well known that molybdenum at high temperatures has high strengthproperties which makes it desirable for high temperature, high stressapplications, e.g. for jet engine turbine blades, glass meltingapparatus, and other structures for use under high temperatures.

However, molybdenum, as such, cannot be advant-ageously utilized becauseof its high rate of oxidation at high temperatures.

In order to protect molybdenum from oxidation at high temperatures,platinum is generally used as a cladding metal and is applied to themolybdenum by bonding a strip or sheet of platinum to the molybdenum orby bonding a tube of platinum onto a core of molybdenum wire or rod toform a composite material. Otherwise, the molybdenum is provided with afitted platinum sheath and the structure formed therefrom is welded onlyat the structural joints. The composite material is unsatisfactory formany applications because of the exposure of the molybdenum at the edgesof the strip, sheet or rod. The sheathed molybdenum is unsatisfactorybecause there is no strong bond between the molybdenum and platinum, andthe platinum is subject to fracture under stresses with the subsequentfailure of the molybdenum.

While the spraying of a thin layer of platinum metal onto molybdenum hasbeen considered, the sprayed layer coating is porous and does notadequately protect the molybdenum.

The present invention relates to the spraying of platinum or otherplatinum group metals such as palladium and rhodium or alloys thereofonto a molybdenum body, or a body of other refractory metal, e.g.tantalum, tungsten, titanium, in a manner whereby the platinum groupmetal completely covers the body with a strong mechanical andmetallurgical bond therebetween and otherwise provides a suitableprotective cladding for use at elevated temperatures.

A simplified flow diagram of the method of the invention is as follows:

Cleaning refractory metal plate Spraying molten platinum on cleanedplate to form a first coating Shot peening the platinum coating Heattreating shot peened coating Spraying a second platinum coating on firstcoating Shot peening second platinum coating Heat treating shot peenedsecond coating It is an object of the invention to provide a method ofcladding a refractory body with a platinum group metal.

It is another object of the invention to provide a method of applying aplurality of sprayed layers of a platinum group metal onto .a refractorybody whereby the body is adequately protected against oxidation at hightemperatures.

Other objects and advantages of the invention will become apparent fromthe description hereinafter following.

In accordance with the invention, a sheet, strip, rod, bar or otherstructure composed of molybdenum is first cleaned by well known chemicalcleaning or etching methods after which molten platinum is sprayed ontothe molybdenum by the known metallizing gun spray method to form a thinlayer coating on the molybdenum. Thereafter, the coating is preferablyplastically deformed, e.g. the coating is shot peened, by known shotpeening procedure, whereby the entire layer is substantially plasticallydeformed sutficiently to condition the layer for improved soliddiffusion of the layer by a subsequent heat treating; and to otherwiseplace the layer and the molybdenum in more intimate contact with eachother. Thereafter, the coated molybdenum is heat treated, i.e. annealed,in a reducing atmosphere, e.g. in dry hydrogen, at a temperature of from600 C. to 1400 C. for about one to ten hours depending upon thetemperature employed, whereby the consequent solid ditfusion of thelayer eliminates the pores inherent in the sprayed platinum layer. Afterthe aforesaid treatment of the first sprayed platinum layer, the coatedmolybdenum is not sufiiciently protected for high temperatureapplication. Therefore, the spraying, peening and heat treating arerepeated whereby a sufficiently thick protective coating is provided bythe spraying in layers with intermediate peening and heat treatment. Theprocess is repeated until the coating has the desired thickness, i.e.from tenths of a thousandth of an inch to any thickness required.Desired thickness is determined by the life expectancy of the endproduct. It has been discovered that while substantially thick spraycoatings may be provided by continuous spraying, such continuouscoatings are inferior to the coatings provided by the aforesaid methodof the invention'and otherwise, the intermediate shot peening not onlyimproves the solid diffusion of the layers under the heat treatment, butalso provides for a stronger mechanical and metallurgical bond betweenthe platinum layer and the molybdenum.

Example A surface of a 2" x 2" molybdenum plate .025" in thickness wascleaned and etched and was sprayed with molten platinum and providedwith a platinum layer 0.003" in thickness. The coated surface was shotpeened and then annealed in dry hydrogen for one hour at a temperatureof 1000" C. The coated plate was cooled in air and the same operationrepeated four times until a layer aprpoximately 0.012" in thickness wasbuilt up on the molybdenum plate. The layer was found to besubstantially non-porous.

While shot peening is specifically referred to, the coating may beplastically deformed by other methods such as pressing or rolling.

While specific embodiments of the invention are described, modificationsare contemplated within the scope of the appended claims:

What is claimed is:

The method of coating a refractory metal body composed of a metalselected from the group consisting of molybdenum, tantalum, tungsten andtitanium, comprising cleaning a surface of the body, spraying a moltenplatinum group metal on the cleaned surface in the form 3 of a thincoat, shot peening the coating, heat treating the shot peened coating ata temperature from 600 C. to 1400 C. to promote solid diffusion of thecoating, and alternately continuing the spraying, shot peening and heattreating until a sufiiciently thick coating'of platinum 5 group metal isbuilt up on the metal body.

References Cited in the file of this patent UNITED STATES PATENTSRobinson Apr. 6, 1937 Colbert et a1 Dec. 16, 1947 Rosenblatt et a1 Oct.4, 1955 Mozley et al June 25, 1957 Wrotnowski Mar. 29, 1960

